Fuel cell systems have been in use for specialty applications (for example, space capsules, sensors) and have been under development for broader applications (for example, stationary power plants, transportation) for many years now. With continued advances, performance has been improved and costs have been reduced such that many of these latter fuel cell systems under development are entering commercial use. However, in order to meet the needs of a less specialized market, these fuel cell systems must be able to handle a wide range of user conditions, ideally with minimal additional complexity to the system. For instance, the ambient temperature and duty cycle can vary widely in different applications. It can be a challenge to meet these requirements, particularly when the application involves frequent storage and start-up in cold conditions.
A particularly attractive fuel cell is the solid polymer electrolyte fuel cell. This type of fuel cell employs an ion conducting membrane as the electrolyte. An individual solid polymer fuel cell generally comprises a membrane electrode assembly (MEA) containing an ion conducting membrane interposed between a cathode and an anode. The ion conducting membrane in the MEA serves as a separator as well as the electrolyte. Catalyst, for promoting the reactions in the fuel cell, is located at the interface between the electrodes and the membrane. Generally, flow field plates are positioned adjacent to each electrode for purposes of distributing the fuel and oxidant reactants to the appropriate electrodes. The flow field plates also typically serve as current collectors, electrode supports, and separators. Since the operating voltage of an individual cell is usually under 1 volt, most fuel cell systems employ numerous cells that are stacked in series to create a higher voltage fuel cell stack.
The electrochemical reactions in a PEM fuel cell proceed more favorably at higher temperatures. However, the operating temperature must be limited in order to prevent damage to the membrane material. The typical operating temperature of a hydrogen-fueled solid polymer fuel cell is under 100° C., which is relatively low compared to other types of fuel cells. Since the electrochemical reaction between fuel and oxidant is exothermic, temperature regulation generally involves cooling of the solid polymer fuel cell stack, hence the temperature regulating subsystem is commonly called the cooling subsystem. (However, the cooling subsystem might also desirably serve as a heating subsystem during cold start-up in order to bring the fuel cell up to the desired operating temperature more quickly.) Solid polymer fuel cell systems are typically liquid-cooled rather than air-cooled especially if higher power densities (power output capability per unit volume) are desired. The reason is that the cooling subsystems typically must shed a significant amount of heat at relatively low temperature (circa 80° C.) with respect to ambient. The use of more efficient liquid-as opposed to air-cooling allows the fuel cell stack cooling channels to be made smaller and hence a lower overall stack volume can be obtained.
In some stationary power applications, a fuel cell system may operate uninterrupted for long periods, albeit at varying power levels. However, more commonly perhaps, a fuel cell system is subjected to frequent on-off cycles and hence it goes through numerous cold starts. For outdoor applications in cold climates, this can mean frequent shutdowns and storage in sub-zero temperatures. The fuel cell system, and particularly the cooling subsystem, must therefore be able to handle repeated storage below freezing without significant degradation. For example, this requirement applies to fuel cell systems for automotive use.
Today's liquid-cooled, internal combustion engine powered automobiles face a similar requirement. To prevent freezing and hence rupturing of the cooling subsystems therein, antifreeze is added to the aqueous coolant. The antifreeze added is typically ethylene glycol but other antifreeze coolants such as propylene glycol, alcohols, and the like can be used. Ethylene glycol transfers heat well, has superior heat capacity, and poses less of a fire hazard (for example, has a flash point greater than 100° C.). Depending on the concentration, an aqueous mixture of ethylene glycol stops the coolant from freezing at temperatures down, for example, to −40° C.
Along with an antifreeze coolant, other additives are used in aqueous automotive and other industrial cooling subsystems in order to slow the corrosion of the metallic components in the coolant circulation loop of the cooling subsystem. For instance, silicates are commonly added to automotive coolants in order to protect aluminum components in the circulation loop. While corrosion is an issue with any aqueous coolant, corrosion can be accelerated by the use of certain antifreeze coolants. Unlike water, ethylene glycol and propylene glycol decompose in the presence of oxygen to form acidic by-products such as glycolic and lactic acids respectively. The presence of these by-products can significantly accelerate corrosion in a coolant circulation loop. Further, the rate of decomposition increases with temperature and in the presence of transition metals. Thus, the high temperatures (circa 200° C.) and metal constructions of conventional automotive coolant circulation loops significantly increase the rate of glycol decomposition and hence corrosion. For this reason, inhibitors (for example, buffers) can also be added to the glycol-based coolants in order to reduce the decomposition of the glycol. Further, cooling subsystems are typically closed (sealed) when operating at temperatures above about 60° C. in order to avoid rapid oxidation of the glycol. More details on this subject can be found in Dow Chemical Company's “Engineering and Operating Guide for Ambitrol Inhibited Glycol-based Coolants”, September 1991.
Historically, glycols, such as ethylene glycol, have been used in alkaline fuel cell systems as the fuel itself. Glycol-based coolants have been suggested for use in the cooling subsystems of certain fuel cell systems. For instance, U.S. Pat. No. 3,507,702 suggests the use of ethylene glycol in the coolant circuit for an aqueous alkali electrolyte fuel cell. Therein, the embodiments and discussion pertain to low voltage fuel cell stacks (for example, 30 V or less) and thus there would be no significant concern about electrical shock hazards through the coolant fluid. There is no discussion regarding corrosion, additives/inhibitors, or removal of ions in the coolant subsystem. Japanese published patent application number 08-185877 discloses an antifreeze coolant system employing ethylene glycol wherein pure water for humidification is obtained via ultrafiltration from the antifreeze coolant. However, no means for maintaining the purity of the antifreeze coolant over time appears to have been provided.
The coolant subsystem in high voltage fuel cell stacks (above about 50 V) can, however, present an electrical shock hazard. If the coolant is sufficiently conductive and is in electrical contact with and interconnects parts of the fuel cell stack that are at different potentials, the coolant fluid can pose a safety problem. Further, the coolant also provides a path for the flow of undesirable corrosion currents. These problems are discussed and addressed in U.S. Pat. No. 3,964,930 wherein various means of electrical isolation (such as coolant tube coatings) are employed in combination with a water-based coolant. A conductivity of less than about 50 μS/cm is stated to be preferred for the water coolant.
Generally, the electrical conductivity of an aqueous coolant increases with the concentration of ions in solution. In some conventional high voltage fuel cell systems, shock and corrosion current concerns are dealt with by using substantially pure de-ionized water as the coolant. An acceptable level for the conductivity of the de-ionized coolant is considered to be of order of 5 μS/cm or less.
Substantially pure, de-ionized water is also desirably used in coolant loops where there is a possibility of the coolant contaminating or damaging MEA components (such as the electrocatalyst and membrane electrolyte) of the fuel cell. Since pure, de-ionized water is fundamentally compatible with the MEA components, fuel cell design and construction may be simplified to allow some contact of the coolant with the MEA components. Note that, even in constructions that attempt to prevent such contact (for example, constructions having isolated piping or redundant seals), there can still be reliability concerns regarding contact resulting from occasional leaks.
Ion exchange resin units and other filters are frequently employed in de-ionized water coolant loops of fuel cell systems to continually remove contaminants and thereby ensure that the water coolant fluid remains substantially free of ionic contaminants. For example, U.S. Pat. No. 5,200,278 discloses a fuel cell system having de-ionized liquid water coolant that is also used for membrane humidification of inlet reactant streams. The water is preferably de-ionized using ion exchange resin units in the loop.
Where tolerance to freezing is required, conventional glycol-based antifreeze coolants containing additives may be used in high voltage, fuel cell systems. However, the coolant subsystem should be reliably isolated electrically and physically from the MEAs in the fuel cell stack, so that electrical shock, corrosion shorting, and contact with the MEA components are not a concern. The use of glycol without additives/inhibitors might be considered as an alternative to isolating the cooling subsystem but it adds to corrosion concerns over those posed by use of water alone, due to the decomposition of glycol into acidic by-products. Consequently, it appears that the use of glycols has been avoided in the coolant of high voltage fuel cell systems that do not have electrically isolated cooling subsystems.
Other antifreeze solvents such as other alcohols and dielectric fluids have been contemplated but these may introduce a significant fire hazard (for example, due to a lower flash point) and/or have poorer heat transfer and capacity characteristics. Instead, solutions have been developed to cope with subzero conditions using pure water coolants, for example, by keeping the system above zero degrees or by removing all water from the system prior to shutdown.